advanced-usage: add relative altitude mini-widget

Documents bluerobotics/cockpit#576

advanced-usage/index.md

@@ -1,7 +1,7 @@
 +++
 title = "Advanced Usage"
 description = "Cockpit advanced usage documentation."
-date = 2023-11-23T19:30:00+11:00
+date = 2023-11-23T20:15:00+11:00
 template = "docs/page.html"
 sort_by = "weight"
 weight = 30
@@ -206,11 +206,11 @@ adding transparent padding at the sides / above+below as necessary
 
 Video recording is possible using a [mini widget](#mini-widgets).
 
-##### Mini Widget Container
+##### Mini Widget Bar
 
-The mini widget container widget is a rectangular container for storing [mini widgets](#mini-widgets).
+The mini widget bar widget is a rectangular container for storing [mini widgets](#mini-widgets).
 
-{{ easy_image(src="mini-widget-container-widget", width=500, center=true) }}
+{{ easy_image(src="mini-widgets-bar-widget", width=500, center=true) }}
 
 #### Mini Widgets
 
@@ -222,6 +222,7 @@ The current options include
 - Vehicle connection status indicator
 - Power / battery indicator
 - Depth indicator
+- (Relative) altitude indicator
 - Very generic indicator
     - clicking on this allows selecting which vehicle variable to track, out of
       any that have been received so far (including custom ones)